Several techniques and systems have been developed for correcting and stabilizing injuries to or malformation of the spine. In one type of system, an elongated member such as an elongate bendable rod is disposed longitudinally along a length of the spine, spanning two or more vertebral levels. In certain applications, the rod is bent to correspond to the normal curvature of the spine in the particular region being instrumented, such as the normal kyphotic curvature of the thoracic region or the lordotic curvature of the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of anchor devices that utilize a variety of fixation elements configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a bone screw that can be threaded into various parts of the vertebrae, particularly the pedicle.
As these systems have evolved, various degrees of freedom of relative orientation were integrated into the systems in order to accommodate misaligned spinal curvature as well as to minimize rod bending and stress risers in the anchor. Presently, the standard in rod-based spinal systems is a polyaxial pedicle screw which includes a yoke having a slot for receiving a bendable elongate connecting rod that is coupled to a threaded bone engaging shank for polyaxial swiveling movement with respect thereto. One example of such a polyaxial screw is shown and described in commonly assigned U.S. Pat. No. 8,162,990, entitled “Multi-axial Spinal Fixation System”, issued on Apr. 24, 2012 to Robert Potash et al. Further advances in the art have now seen anchor systems that include movement of the polyaxial rod-receiving yoke in an additional plane of motion relative to the bone fastener. Such a system is shown, for example, in U.S. Pat. No. 8,449,578, entitled “Multiplanar Bone Anchor System”, issued on May 28, 2013 to Matthew L. Kaiser et al.
Nevertheless, improvements in bone anchoring systems that increase the flexibility of accommodating different spine angulations and curvatures as well as minimizing bending of connecting rods and reducing complexity and cost of the anchor are desirable.